The more serious the side effects of a treatment, the more evidence you should require that the treatment will actually help you before you are willing to accept that treatment. So the worse the side effects, the higher the cut-off value. Conversely, the less serious the side effects, the less evidence you should require before accepting the treatment.
Many alternative treatments are less toxic or non-toxic and many conventional treatments are highly toxic - but you should be aware that there are many exceptions in both areas - you need to get specific information on the treatments you are considering.
The mere fact that a treatment is non-toxic does not mean it should be preferred over one that is highly toxic -- there are many non-toxic treatments which just don't work -- so in my view the toxicity is only a modifying factor in making the decision. Specifically if there is a treatment which is proven to work but is toxic, it would be a terrible mistake to use a treatment which is non-toxic but which also has little real evidence for it.
Some complementary treatments such as mind-body approaches, exercise, or a healthy (but not fad) diet have inherent benefits instead of side effects. You can (mathematically, but perversely) think of these benefits as "negative side effects"! Clearly, given two treatments with the same promise, one would prefer the treatment which has inherent benefits regardless of whether it turns out to be effective against the cancer. My impression is that most general health enhancing therapies are not particularly powerful against the disease itself and should be used with other approaches - as an overall package they may increase the odds while increasing quality of life at the same time.